The present invention relates to a method and apparatus for applying a layer of coating material to a substrate, in general, and to the application of an extremely thin, constant thickness coating layer or coating layer pattern of such material to the surface of a moving web, in particular.
In many coating applications, extremely thin, constant thickness coatings are absolutely essential in order to avoid degrading the performance of the coated device and/or the equipment with which such coatings are employed. In, for example, audio and video magnetic tapes, if the magnetic media coating is excessively thick or there are significant variations in coating thickness, magnetic coupling and therefore information transferal between the magnetic media in the coating and, for example, the read/write head of audio or video recording or reproducing equipment in which the tape is utilized could be substantially degraded because of the increased spacing or the spacing variations between these components that necessarily result when such coatings are employed.
A number of coating techniques presently exist for applying coating materials to a web or other object's surface. Many of these techniques employ an electrostatic field between a coating applicator and a web or object surface spaced therefrom to assist in both the uniform and efficient deposition of coating material on such a surface. In, for example, the well-known process of electrostatic spray painting, an electrostatic field is established between an electrically conductive grid and a metal object spaced therefrom. The electrostatic field is created by a relatively high DC voltage (100,000 V) connected between grid and object, with the object ordinarily being spaced several feet from the electrified grid. Air pressure supplied to a reservoir of coating fluid coupled to one or more coating applicator orifices causes coating fluid droplet formation at the output of said orifices. The droplets are subsequently propelled into the electrified grid by air pressure generated forces where they become electrostatically charged and then deposited, in layer form, on a metal object surface by forces associated with the electrostatic field. Unfortunately, due to the relatively large droplet size generated by this type of coating apparatus, the resulting coating layer is well in excess of a thickness level that would avoid the above-mentioned problem associated with excessively thick magnetic media coatings.
Electrostatic coating apparatus capable of generating and subsequently depositing relatively small coating material particles on a substrate has been described in copending and commonly assigned U.S. patent application Ser. No. 32,606, by S. KISLER, filed Apr. 1, 1987. In this particular application, a method and apparatus for electrostatically coating articles are disclosed that are capable of applying an extrexely thin and virtually constant thickness coating to a substrate. The method and apparatus include a coating applicator having an opening formed of dielectric material through which coating material flows for substrate coating purposes. The coating material supplied to the applicator is pressurized to maintain a substantially constant volume of coating material at the applicator opening output and is vibrated in the vicinity of the applicator opening to provide a uniform flow of coating material to the applicator output, to stabilize the shape of the coating material at the applicator output and to preclude changes in coating material flow rate caused by electrostatic field induced coating material drying. An electrostatic field of predetermined magnitude is established between the coating material surface and an adjacent substrate to thereby extract minute, uniform sized particles from coating material at the applicator opening and subsequently deposit an extremely thin layer of these particles on the adjacent substrate as it is moved past the applicator opening. While this apparatus is effective in applying a relatively thin layer of coating material to a substrate, the rate at which coating material can be applied with such apparatus is limited and it is relatively difficult to maintain the required volume of coating material at the applicator opening output. In addition, multiple nozzles must be employed when coating very large width webs (e.g. a web having a width of 60 inches). When such a nozzle arrangement is employed, the same pressure must be maintained at the output of each nozzle which is a relatively difficult condition to achieve.
It is a primary cbject of the present invention, therefore, to provide coating apparatus that is capable of applying a constant thickness layer of coating material to a substrate over a wide range of coating material flow rates and over a wide range of coating layer widths.
It is another object of the present invention to provide coating apparatus of reduced comp1exity that is capable of applying a layer of coating material to a substrate at a constant coating material flow rate.
It is a further object of the present invention to provide coating apparatus that is capable of applying a relatively thin layer of coating material to a substrate in one or more selected coating patterns.
Other objects, features and/or advantages of the present invention will be readily apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.